1. Field of the Invention
The invention relates to the field of polymer chemistry and relates to sulfonated polyarylene compounds, such as can be used, for example, in ion exchange membranes in fuel cells or other electrochemical processes or as low-fouling membranes for nanofiltration, ultrafiltration or microfiltration or reverse osmosis, as well as a method for the production thereof and use thereof.
2. Discussion of the Background Information
Among the known fuel cell types the acid polymer electrolyte fuel cell (PEM) is characterized by a high power density and a favorable ratio of power to weight. Membranes of poly(perfluoroalkyl sulfonic acids) such as, e.g., Nafion® (DuPont) are currently considered to be standard membranes due to their high chemical stability and high conductivity. The disadvantages of these membranes are the limitation of the operating temperature to a maximum of 80-90° C. and the currently still very high price. However, a higher operating temperature is desirable, since the electrode kinetics are favorably influenced, catalyst poisons such as carbon monoxide desorb more quickly and the cooling requires a lower expenditure (C. Wieser, Fuel Cells 4, 245 (2004)). Furthermore, the poly(perfluoroalkyl sulfonic acids) have a high methanol permeability, which excludes them from use in DMFC (direct methanol fuel cells).
Sulfonated polyaryl compounds are being intensively studied as alternative materials for fuel cell membranes. Poly(ether ketone)s (PEK, PEEK) and poly(ether sulfone)s (PES, PEES) are particularly favored thereby. Other sulfonated high-performance polymers such as polyimides have likewise been examined (N. Cornet et al., J. New Mater. Electrochem. Syst. 3, 33-42 (2000)). However, these are not suitable for use as fuel cell membranes in terms of their chemical stability (G. Meier et al., Polymer 47, 5003-5011 (2006)).
Sulfonation is carried out in most cases at the main chain on electron-rich aromatics. However, it is generally known that the sulfonation of aromatics is reversible and is promoted by electron donor substituents (e.g., ether groups), high temperatures and acidic media (Organikum, 22 ed., Wiley-VCH Verlag GmbH Weinheim 2004, ISBN 3-527-31148-3). Furthermore, it has been reported that the sulfonation of the polymer main chain leads to a destabilization and thus to a reduction of the molar mass and reduction of the mechanical properties (J. F. Blanco et al., J. Appl. Polym. Sci. 84, 2461-2473 (2002)).
On the other hand, it was found that the sulfonic acid group in poly(styrene sulfonic acid) is resistant to hydrolysis in water up to approx. 200° C. (C. Vogel et al., Fuel Cells, 4, 320-327 (2004)). However, poly(styrene sulfonic acid) cannot be used in fuel cells due to its oxidation sensitivity (J. Yu et al., Phys. Chem. Chem. Phys. 5, 611-615 (2003)).
Polymer fuel cell membranes with sulfonated aromatic side chain have likewise been described. The object thereby is to support the microphase separation between hydrophobic main chain and hydrophilic elements (sulfonic acid groups). It is expected that the swelling of the membranes in water will be minimized by the inserted spacers between polymer main chain and sulfonic acid group.
Lafitte et al. converted a polysulfone on bisphenol-A basis with the cyclic anhydride of the 2-sulfobenzoic acid and other electrophiles in a two-stage and multiple-stage reaction (B. Lafitte et al., Macromol. Rapid Commun. 23, 896-900 (2002) and ibid 26, 1464-1468 (2005)). In the first stage, an activation of the polymer occurs through conversion with butyllithium. The second stage comprises the conversion of the activated polymer with 2-sulfobenzoic anhydride or fluorobenzyl chloride. The latter is converted in a further step in a nucleophilic reaction with a hydroxyaryl sulfonic acid. The disadvantage of this method is the sensitivity of the reactants (BuLi, activated polymer) to contaminants (e.g., water, oxygen, carbon dioxide) and the subsequent reactions associated therewith and the poor control of the degree of sulfonation and the relatively high price of butyllithium.
Chen et al. described polyimide sulfones with sulfobenzoyl side groups (S. Chen et al., Polymer 47, 2660-2669 (2006)). In this study, dichlorodiphenyl sulfone was first carboxylated in a multiple-stage reaction (BuLi+CO2), then converted into the acid chloride and converted by a Friedel-Crafts acylation with benzene to the benzoyl derivative. The sulfonation of this monomer was carried out with oleum (30% SO3) at 75° C. The sulfonated polymers are obtained via further conversions with 1.) aminophenol and 2.) naphthalenetetracarboxylic acid anhydride. This method is very complex and cost-intensive, especially as a total yield of only 50% was achieved in the production of the sulfonated monomers. Furthermore, it is known that polyimides are not stable under fuel cell conditions (G. Meier et al., Polymer 47, 5003-5011 (2006)).
Ghassemi et al. described a block copolymer based on poly(phenylene sulfone) and poly(4′-phenyl-2,5-benzophenone) that is sulfonated exclusively in the phenyl benzophenone side chain (H. Ghassemi et al., Polymer 45, 5855-5862 (2004)). No information was given on the hydrolytic stability of these products.
Gieselmann et al. presented side groups sulfonated polybenzimidazoles (M. Gieselman et al., Macromolecules, 25, 4832-4834 (1992)) in order to improve the solubility of these polymers. The modification was carried out by alkylation of the NH nitrogen of the imidazole ring in a two-stage reaction 1.) deprotonation of the NH nitrogen with a strong base (LiH) and subsequent conversion with 1,3-propane sultone or benzyl bromide sulfonic acid.